Many factors make genetically complex diseases complex. Classically they are defined as an interaction between multiple genetic variants and non-genetic factors. Progress in genome sequencing and within-species variation has generated much interest in identifying polygenic variants from human and model organisms, with some success. But one cannot lose sight of the importance of physiological complexity; even Mendelian variants can wreak havoc when operating in a complex biological system. For functional phenotypes, such as excitability disorders of the CNS, this concept is understudied. Epilepsy is genetically complex to be sure, but as the canonical excitability disorder of the brain it also serves as a leading example for approaching other, harder-to-crack functional disorders, such as autism and schizophrenia, that are also likely to have excito- pathology at their cores. Neuronal excitability is determined primarily by molecules, such as ion channels and transporters, neurotransmitter receptors, and synaptic proteins, controlling membrane potential and synaptic signaling in order to achieve an appropriate balance of excitation and inhibition. Although cis-variants in genes encoding these molecules can lead to specific phenotypes, trans-factors that regulate their expression must be critical for maintaining this balance at a higher, coordinated level. We previously identified and characterized hypomorphic and null genotypes in Celf4 (formerly known as Brunol4), encoding a brain-specific member of the BRUNO/CUGBP/CELF family of RNA binding proteins. Celf4 mutants have a complex seizure disorder and other neurological phenotypes, such as hyperactivity, mild obesity and abnormal social interaction. Very recently human CELF4 deficiency revealed these and additional symptoms such as intellectual disability. In our initial funding period, we found that CELF4 is most tightly associated with very high-density RNA granule particles and targets a vast number of mRNAs in excitatory neurons. Many targets are involved in synaptic functions, and they tend to be dysregulated within neurons of mutant mice - in all directions, but with a tendency towards increased expression away from the cell body. These findings are consistent with a role for CELF4 in control translational silencing at local, subcellular levels. We also obtained evidence for CELF4 effects on intrinsic neuronal hyperexcitation, via increased expression of sodium channel Nav1.6, and system- wide dysregulation via impaired homeostatic plasticity; the combination of the two presumably underlay full- blown disease. In the next five years we will examine in greater detail several key aspects of the molecular function of CELF4, including a greater precision/ depth examination of the fate of CELF4 target mRNAs when CELF is depleted and a first look at the protein composition of CELF4-containing ribonucleoprotein particles. These studies will allow us to flesh-out the role of CELF4 and related proteins in translational silencing. In parallel, we will explore whether CELF4 has a coordinating role in shaping cellular responses by examining mutant cell culture, acute brain slice and whole animal models of neuronal plasticity.